1. Technical Field
The present invention relates to a spark plug, and more particularly, to a spark plug in which a tip is provided on at least either of a center electrode and a ground electrode.
2. Related Art
A spark plugs used to introduce the ignition energy into a combustion chamber of an internal combustion engine as of a motor vehicle generally includes a cylindrical metal shell, a cylindrical insulator which is disposed in an internal hole of the metal shell, a center electrode which is disposed in an internal hole at a front end side of the insulator, and a ground electrode which is joined to a front end side of the metal shell at one end and which defines a spark discharge gap at the other end between the center electrode and itself. Then, in the spark plug, an ignition spark is discharged in the spark discharge gap defined between a front end portion of the center electrode and a distal end portion of the ground electrode within the combustion chamber to ignite an air-fuel mixture filling the combustion chamber for burning.
An Ni alloy is generally used as a material for forming a center electrode and a ground electrode. Although the Ni alloy is slightly inferior with respect to oxidation resistance and wear resistance to a noble metal alloy which contains a noble metal such as Pt and Ir as a main composition, the Ni alloy is inexpensive compared with the noble metal and is therefore used preferably as the material for forming the central electrode and the ground electrode.
In recent years, in order to achieve high outputs and to enhance the fuel economy of engines, there is a tendency to increase the temperature in combustion chambers. In addition, there have now been used an engine in which a discharge portion which forms a spark discharge gap protrudes into an interior of a combustion chamber so as to improve the ignition performance thereof. In these situations, the discharge portion of the spark plug are exposed to high temperatures, which tends to promote the facilitation of oxidation wear of a center electrode and a ground electrode which define the discharge portion. Then, there have been developed methods for suppressing the oxidation wear of the center electrode and the ground electrode by providing tips individually on a front end portion of the center electrode and a distal end portion of the ground electrode which face each other and causing a spark discharge to occur at the tips.
For example, JP-A-9-7733 describes therein an “internal combustion engine spark plug . . . a noble metal tip is joined to a discharging location of a front end portion of the center electrode and/or a distal end portion of the ground electrode, wherein the noble metal tip is made of an Ir—Rh alloy with a quantity of Rh added ranging from 1 wt % to 60 wt %” (refer to claim 1 of JP-A-9-7733). It is disclosed that in the noble metal tip of the internal combustion engine spark plug, the wear resistance is improved by Ir having a high melting point, and the volatilization and wear of Ir at high temperatures are prevented by adding Rh to Ir (refer to paragraph 0022 in JP-A-9-7733).
Japanese Patent No. 4402046 describes therein a “spark plug . . . the noble metal member contains Ir as a main composition, 6.5 mass % or more and 43 mass % or less of Rh, 5.2 mass % or more and 41 mass % or less of Ru and 0.4 mass % or more and 19 mass % or less of Ni” (refer to claim 1 of Japanese Patent No. 4402046). Then, Japanese Patent No. 4402046 discloses the following facts about the noble metal member of the spark plug (refer to paragraphs 0011 and 0012 of Japanese Patent No. 4402046). Since Ir having a high melting point is contained as the main composition, the good heat resistance is exhibited. Since the predetermined quantity of Rh is added, the volatilization and wear of Ir can be suppressed even at high temperatures. Since the predetermined quantity of Ni is added, an abnormal scooped wear which is generated from time to time in noble metal members depending on service conditions can be suppressed. Since the predetermined quantity of Ru is added, the wear of the noble metal member and the occurrence of a sweat-out phenomenon in which particulate matters adhere to the noble metal member can be suppressed. Additionally, the Ru addition can suppress further the occurrence of a separating phenomenon which results from the progression of the sweat-out phenomenon.
JP-A-11-154583 aims at providing a spark plug in which wear triggered by oxidation and volatilization of an Ir composition is made difficult to occur, thereby exhibiting superior durability (refer to paragraph 0004 in JP-A-11-154583). JP-A-11-154583 describes a spark plug wherein a firing portion which defines a spark discharge gap is made mainly of Ir, an area where the Vickers hardness becomes 400 Hv or less is formed to a depth or thickness of 0.05 mm from a surface of the firing portion, and a mean value of dmin/dmax which is a ratio of a minimum diameter dmin to a maximum diameter dmax of particles appearing on a section when a sectional structure of the area is observed is 0.7 or more (refer to Claim 1 and 2 of JP-A-11-154583). In a tip produced by plastically forming a metallic material made mainly of Ir through rolling, cutting, punching and the like, strain remains in the metallic material to some extent and is hence hardened as a result of the plastic forming. The hardness is increased relatively high particularly in a surface layer portion area where the strain remains to a large extent. In the event that a firing portion is formed by using the tip formed in the above-described way, wear triggered by oxidation and volatilization of the Ir composition is progressed easily. Then, it is disclosed in JP-A-11-154583 that the tip is annealed at 900 to 1700° C. to be softened so that a surface layer portion area having a predetermined thickness is formed where the Vickers hardness becomes 400 Hv or less, whereby the oxidation and volatilization of the Ir composition are suppressed effectively (refer to paragraphs 0008 to 0010 in JP-A-11-154583). In addition, particles in the tip metallic material which is subjected to the plastic forming and is hence hardened are largely stretched in the forming direction, and the dmin/dmax shows a quite small value. However, it is also disclosed that when the tip metallic material is annealed in the above-described way, recrystallization is progressed, and the dmin/dmax is gradually increased, whereby the oxidation and volatilization of the Ir composition in the firing portion are suppressed further effectively (refer to paragraph 0012 in JP-A-11-154583).
JP-A-2010-218778 describes an internal combustion engine plug electrode material having a pillar-like crystal which extends over the length of a tip and in which a hardening rate [(hardness in Hv after forming)/(hardness in Hv after heat treatment at 1100° C. for 20 hours which simulates plug service conditions)×100 (%)] which is a ratio of a hardness after forming to a hardness after the heat treatment at 1100° C. for 20 hours which simulates plug service conditions is 130% or less (refer to Claims 1 and 2 in JP-A-2010-218778). As an internal combustion engine plug electrode material in which the suppression effect of high temperature oxidation wear is improved, it is described that “it is necessary that crystalline grains are bulky and have an elongated shape and that no forming strain remains therein so that the recrystallization does not progress therein under its service temperature conditions.” (refer to paragraph 0011 in JP-A-2010-218778).